A known industrial process for the conversion of hydrogen sulphide separated from a gas stream is the so-called Claus process. Hydrogen sulphide is first separated from the remainder of the gas stream by a solvent extraction process. After solvent regeneration, a low-pressure H2S-rich gas is obtained which is dealt with in the Claus process. About one third of the hydrogen sulphide in this gas is oxidized with air to sulphur dioxide in a burner, according to:2H2S+3O22H2O+2SO2  (1)
The sulphur dioxide subsequently reacts with the remaining hydrogen sulphide to elemental sulphur according to the Claus reaction:2H2S+SO22H2O+3/nSn  (2)
The H2S conversion is about 70%. In order to achieve a H2S conversion of more than 70%, several catalytic Claus reaction steps at a temperature above about 200° C. are needed and sulphur has to be condensed in between the reaction steps. Due to thermodynamic limitations, the H2S conversion of the Claus process is at most 97%. The remaining hydrogen sulphide is incinerated to sulphur dioxide or treated in a Claus tail gas treating process, such as the SuperClaus process or the Shell Claus Offgas Treating (SCOT) process.
Disadvantages of the Claus process are that several reaction steps are needed, the overall H2S conversion is at most 97%, and the reaction rate is low, due to the low pressure of the reactant gas. Moreover, the hydrogen sulphide has first to be separated from the remainder of the gas. It would be advantageous if hydrogen sulphide could be selectively oxidized, i.e. without the need to separate it from the remainder of the gas.
In U.S. Pat. No. 4,886,649 a process for the selective oxidation of hydrogen sulphide to elemental sulphur in a hydro carbonaceous gas is disclosed. H2S is oxidized according to the following reaction:2H2S+O22H2O+2/nSn  (3)
The reaction (3) is performed in two stages in U.S. Pat. No. 4,886,649. Oxidation within the first stage is carried out in a fluidised bed of a granular catalyst containing 10-20% by mass of magnesium chromate on aluminium oxide at temperatures between 250-350° C. In the second oxidation stage, the unreacted hydrogen sulphide and oxygen from the first stage are reacted at 140-155° C. in the presence of a catalyst containing vanadium pentoxide and aluminium oxide.
A disadvantage of the process of U.S. Pat. No. 4,886,649 is that gaseous sulphur is formed in the first stage. At the concentrations wherein sulphur is present in the gaseous effluent of the first stage, this will inevitably result in the formation of a sulphur mist, which is difficult to separate from the gas flow and results in deposition of elemental sulphur on the catalyst, reactor elements or conduits. Another disadvantage is that some sulphur dioxide will be formed at the process temperature of the first step.
In U.S. Pat. No. 4,311,683 is disclosed a process for the removal of hydrogen sulphide from a feed gas, and the production of sulphur therefrom, by selective oxidation of the H2S with oxygen. The feed gas stream comprising H2S and oxygen is passed through a catalyst bed under conditions such that the hydrogen sulphide and oxygen react to produce elemental sulphur vapour. The inlet temperature into the catalyst bed is between about 250° and 450° F. (121° and 232° C.). In the examples, this temperature is at least 325° F. (163° C.). The catalyst is an oxidation catalyst comprising an oxide and/or sulphide of vanadium supported on a non-alkaline porous refractory oxide. It is mentioned that sulphur deposition and consequent catalyst deactivation are prevented by maintaining the partial pressure of free sulphur in the oxidation reactor below that necessary for condensation. Preferably, the temperature is maintained below 450° F. (232° C.) and the H2S concentration in the feed is kept low by diluting the feed with an inert gas or with recycle gases.
In U.S. Pat. No. 6,207,127 is disclosed a method for making a catalyst for the selective oxidation of hydrogen sulphide into elemental sulphur. The catalyst comprises a mixed oxide of iron and zinc on a silica support. The catalyst is used in a selective oxidation process in an adiabatically operating reactor wherein the inlet temperature of the catalyst bed is at least 150° C., preferably at least 170° C., i.e. above the dew point of the sulphur formed.
In the processes of U.S. Pat. No. 4,311,683 and U.S. Pat. No. 6,207,127, sulphur is kept in the vapour phase by performing the selective oxidation at temperatures above about 160° C. and by keeping the sulphur concentration very low. This means that these processes are not suitable for deep desulphurisation of gases having a high content of hydrogen sulphide, since this would inevitably lead to sulphur deposition.
There is a need in the art for a process for the direct selective oxidation of hydrogen sulphide, that is suitable for the deep desulphurisation of gaseous streams with a relatively high H2S content, i.e. above 0.5 vol % and up to 25-50 vol %, wherein deposition of solid or polymerized sulphur on reactor elements, conduits or the catalyst is prevented and wherein the formation of sulphur dioxide is minimized.